For radio frequency communication systems, especially those operating in unlicensed bands, there are two major reasons for errors, namely noise and interference. To counteract noise and interference, there are two main approaches which are principally different. The first one is to use forward error correcting coding FEC, which essentially means that redundancy is added to the information at the transmitter side. The added redundancy can be used by the receiving device to correctly decode the information. The second approach is to use automatic retransmission request (ARQ), which means that in case the receiver is not able to decode the information, it requests a retransmission.
These two main alternatives, FEC and ARQ, can also be combined which is referred to as hybrid ARQ. In addition, in case of retransmission, it is also possible to retransmit other information which can aid in the decoding.
Whether FEC or ARQ is the most suitable approach depends on the channel conditions. If the link is limited by noise, i.e. the received signal strength is too weak, then FEC is typically preferable. On the other hand, if the major problem is burst-like interference, then retransmission using ARQ is typically more effective. In particular, when operating in the Industrial, Scientific and Medical band (ISM-band), where the likelihood for burst-like interference is high, applying FEC may actually worsen the problem. The reason for this is that applying FEC means that the data packet increases in length and the probability that some part of the packet is interfered is therefore increased. Because the interference is often very strong, the FEC will not be powerful enough to help.
Since FEC is efficient in case of noise, but might degrade the performance in case of interference, it is essential to know whether the link is noise limited or interference limited in order to decide whether to increase or decrease the amount of coding.
The existing solutions operate so that the device which determines which modulation and coding scheme to apply does this by estimating whether noise or interference is the main cause for the errors. As this is based on an estimation, finding a suitable coding and modulation scheme may therefore take some time, making it less useful for short messages and also making it less effective when the channel conditions are rapidly varying
The approach taken today is that the transmitting devices selects a coding and modulation scheme and sometimes alters the scheme to see if another set of parameters might actually give better performance, possibly based on an estimation.
In addition to changing the modulation and coding scheme, it is also possible to adjust the length of the packet. This is also done by the transmitter side based on estimation rather than on explicit information from the receiver side.
Therefore, there is a need for a radio frequency communication device being able to adapt, quickly and without additional cost, to both interference and noise.
Furthermore, the contemporary technology relies heavily on resending message packages. With the exception for broadcast standards, virtually all standards for wireless communications support some kind of retransmission scheme. When a data packet is not correctly received, the receiving device requests the transmitting device to retransmit information related to the erroneous packet. The idea is that the channel conditions hopefully would be better for the retransmitted packet. In some systems a packet is resent and is to be combined with the previously sent packet to increase the chances of a correct decoding. In other systems, the resent packet is encoded differently to increase its chances of being decoded correctly.
Irrespectively which of the above approaches that is taken, the packet used for retransmission is at least of the same length and transmitted at the same power. This means that in case of retransmission, twice the amount of channel resources are needed. The retransmission is thus somewhat wasteful in terms of power, channel allocation, etc., which in case of an interference limited system means that more interference than necessary is created towards the other users.
The waste of resources is further increased if also the reception of the retransmitted packet is unsuccessful, as then yet another retransmission might be performed.
This leads to that many times an unnecessary amount of resources is used for the retransmission. Specifically, often the original packet is very close to being correctly decoded and just a slight additional amount of information is needed, not a retransmission of a packet of the same length as the original one. This means that the transmission is wasteful in terms of used resources.
When the systems is interference limited, if several of the users are using the transmission medium more than necessary this can lead to significant performance degradation.
There is thus a need for a manner of resending data packets that does not cause wasteful use of resources.